Use python discovered through env instead of hardcoding a path

This is more flexible regarding the location of the python binary
but doesn't allow to pass the -u flag. The -i flag can be passed
from inside the script using the PYTHONINSPECT env variable.

This fixes a problem with the esx_vi_generator.py on FreeBSD.

generator.py

@@ -1010,7 +1010,7 @@ def buildWrappers():
     classes = open("libvirt.py", "w")
 
     extra = open(os.path.join(srcPref,"libvirt-override.py"), "r")
-    classes.write("#!/usr/bin/python -i\n")
+    classes.write("#!/usr/bin/env python\n")
     classes.write("#\n")
     classes.write("# WARNING WARNING WARNING WARNING\n")
     classes.write("#\n")

libvirt-override.py

@@ -2,6 +2,11 @@
 # Manually written part of python bindings for libvirt
 #
 
+# Specify -i commandline option after python was started
+if __name__ == "__main__":
+    import os
+    os.environ["PYTHONINSPECT"] = "1"
+
 # On cygwin, the DLL is called cygvirtmod.dll
 try:
     import libvirtmod